This invention relates to electronic filtering circuits for sampled-data systems and, more particularly, it relates to a prefilter or cosine filter for a switched capacitor type filter.
In sampled-data systems or devices where the input signal frequency (F.sub.i) is in a range that encompasses or is close to the sampling or clock frequency (F.sub.c), spurious frequencies can be created by the aliasing or overlapping of signal spectrum giving (F.sub.c -F.sub.i) and (F.sub.c +F.sub.i) components. One proposed solution to this problem for circuits using charge coupled devices (CCD) is described in IEEE Journal of Solid State Circuits, Vol. SC-12, No. 6, December, 1977. However, the proposed circuitry and its implementation with CCD devices is relatively complex and is not applicable to circuits using metal-oxide-silicon (MOS) devices and employing switched capacitor filters where the same problem existed prior to the present invention.
A switched capacitor filter is a sampled-data system that samples the analog input signal at a given rate. If the signal is not band-limited, all components around the sampling frequency will be folded back into the base-band, causing the aforesaid aliasing problem. In a switched capacitor low-pass filter clocked at (F.sub.s), the incoming signal has to be band-limited to (F.sub.s -F.sub.p) by an analog prefilter where (F.sub.p) is the pass-band edge. If (F.sub.s) is low, a complex analog prefilter would be required. However, if the filter is preceded by a cosine filter, the incoming signal can be band-limited to 2 (F.sub.s -F.sub.p) externally which relaxes the requirement on the external analog anti-aliasing filter by a factor of 2. It thus becomes necessary to provide circuitry compatible with a switched capacitor filter that would provide the aforesaid cosine filtering function.